Fold interference pattern in thick-skinned tectonics; a case study from the external Variscan belt of Eastern Anti-Atlas, Morocco

Accepted Manuscript Fold interference pattern in thick-skinned tectonics; a case study from the External Variscan Belt of Eastern Anti-Atlas, Morocco L. Baidder, A. Michard, A. Soulaimani, A. Fekkak, A. Eddebbi, E.-C. Rjimati, Y. Raddi PII:

S1464-343X(16)30119-4

DOI:

10.1016/j.jafrearsci.2016.04.003

Reference:

AES 2539

To appear in:

Journal of African Earth Sciences

Received Date: 12 November 2015 Revised Date:

1 April 2016

Accepted Date: 2 April 2016

Please cite this article as: Baidder, L., Michard, A., Soulaimani, A., Fekkak, A., Eddebbi, A., Rjimati, E.-C., Raddi, Y., Fold interference pattern in thick-skinned tectonics; a case study from the External Variscan Belt of Eastern Anti-Atlas, Morocco, Journal of African Earth Sciences (2016), doi: 10.1016/ j.jafrearsci.2016.04.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Fold interference pattern in thick-skinned tectonics; a case study

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from the External Variscan Belt of Eastern Anti-Atlas, Morocco L. Baiddera, A. Michardb, *, A. Soulaimanic, A. Fekkakd, A. Eddebbi c,

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E.-C. Rjimatie, Y. Raddie

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Hassan II University, Faculty of Sciences Aïn Chock, Geosciences Laboratory, BP 5366 Maârif, Casablanca, Morocco

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b

Pr. Em. University of Paris-Sud, 10, rue des Jeûneurs, 75002 Paris, France

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Department of Geology, Faculty of Sciences-Semlalia, Cadi Ayyad University, P.O. Box 2390, Marrakech, Morocco

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Chouaïb Doukkali University, Faculty of Sciences, Earth Sciences Department, B.P. 20, 24000 El Jadida, Morocco

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Direction de la Géologie, Ministère de l'Energie et des Mines, B.P. 6208, Rabat Instituts Haut Agdal, Rabat, Morocco

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Abstract

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Conflicting views are expressed in literature concerning fold interference patterns in thick-

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skinned tectonic context (e.g. Central Anti-Atlas and Rocky Mountains-Colorado areas). Such

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patterns are referred to superimposed events with distinct orientation of compression or to the

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inversion of paleofaults with distinct strike during a single compressional event. The present

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work presents a case study where both types of control on fold interference are likely to be

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combined. The studied folds occur in the Tafilalt-Maider area of eastern Anti-Atlas, i.e. in the

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E-trending foreland fold belt of the Meseta Variscan Orogen in the area where it connects

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with the SE-trending, intracontinental Ougarta Variscan belt. Detail mapping documents

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unusual fold geometries such as sigmoidal and croissant- or boomerang-shaped folds

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associated with a complex major fault pattern. The folded rock material corresponds to a 6-8

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km-thick Cambrian-Serpukhovian sedimentary pile that includes alternating competent and

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incompetent formations. The basement of the Paleozoic succession is made up of

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ACCEPTED MANUSCRIPT rhomboedric tilted blocks that formed during the Cambrian rifting of north-western

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Gondwana and the Devonian dislocation of the Sahara platform. The latter event is

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responsible for an array of paleofaults bounding the Maider and South Tafilalt Devonian-

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Early Carboniferous basins with respect to the adjoining high axis. The Variscan Orogeny

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began during the Bashkirian-Westphalian with a N-S direction of shortening that converted

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the NW-trending Ougnat-Ouzina paleogeographic high into a mega dextral shear zone. Folds

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developed on top of a moving mosaic of basement blocks, being oriented en echelon along the

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inverted paleofaults or above intensely sheared fault zones. However, a dominantly NE-SW

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compression responsible for the building of the Ougarta belt also affected the studied area,

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presumably during the latest Carboniferous-Early Permian. The resulting fold interference

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pattern and peculiar geometries would exemplify a dual control of deformation by both the

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variably oriented basement paleofaults and the evolution of the regional shortening direction

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with time.

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Keywords: Thick-skinned tectonics, Superimposed folding, Inversion tectonics, Variscan Belt,

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Anti-Atlas, Ougarta.

1. Introduction

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The fold geometry resulting from the superposition of folds of similar type has been the

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object of numerous classical studies. John Ramsay (1962) first described the interference

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patterns produced by two successive foldings with different relative orientation of their shear

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and flattening directions. However, he basically considered small scale natural examples of

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such superimposed folds. In contrast, Jean Goguel (1937, 1939) and Marcel Lemoine (1972)

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described examples of superimposed folding at map scale in south-eastern France where the

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ACCEPTED MANUSCRIPT Alpine folds (Miocene) superimpose the Pyrenean-Provençal ones (Late Eocene). In their

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works, the superposition of two folding events resulted in fold tightening (when shortening

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directions were similar) and in the formation of large, arcuate fold systems. Folding occurred

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there in a Mesozoic-Cenozoic sedimentary sequence detached above a thick Triassic evaporite

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basin (Le Pichon et al., 2010; Andreani et al., 2010).

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Investigating the fold relationships with basement faults in the very different context of

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the Laramide Rocky Mountains, Mitra and Mount (1998) showed that the orientation of the

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basement-cored folds is directly controlled by that of the reverse fault underneath. Looking at

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the same region, Marshak (2000) insisted on the following corollary: the initial rifting pattern

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of the basement and the concept of fault inversion (Cooper and Williams, 1989; Turner and

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Williams, 2004) are critical for the interpretation of thick-skinned tectonics. As soon as the

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basement has been affected by two distinct sets of paleofaults during its early rifting

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evolution, then their inversion will result in two distinct, but possibly coeval directions of fold

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axis in the frame of a single regional compression.

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2. The Anti-Atlas Paleozoic fold belt (Fig. 1) is a typical example of dominantly thick

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skinned belt (Burkhard et al., 2006) that extends at the south-western front of the

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Alleghanian-Variscan (Hercynian) orogen in southern Morocco (Soulaimani and

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Burkhard, 2008; Michard et al., 2010). Its western-central part (Akka-Tata area) offers

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excellent examples of interference between two sets of flexural-slip folds with differently

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oriented axes associated with faulted basement inliers. This interference pattern received

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contradictory interpretations. According to Faik et al. (2001), it would result from one

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single compressional event (i.e. the main, NW-oriented Variscan compression) acting on a

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formerly rifted basement with two sets of faults with different strike. The authors argue

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that the observed, E-trending folds would have formed prior to the interfering NE-trending

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ones. In contrast, Caritg et al. (2004) argue that the dome-and-basin structures of the Tata

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ACCEPTED MANUSCRIPT area are typical for the class 1 or 1-2 interferences as defined by Ramsay and Huber

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(1987), involving a first generation of SW-NE open folds superimposed by a second

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generation with similar style and wavelengths trending in an E-W direction. Albeit they

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clearly recognize the control of folding by the inversion of basement paleofaults with

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different strike, Caritg et al. (2004) as well as Helg et al (2004) conclude that a rotation of

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the compressional stress occurred in the area during the Variscan orogeny. In the present

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paper, we present another case study from the Tafilalt-Maider area of easternmost Anti-

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Atlas, which is the very specific area where the ENE-trending Anti-Atlas belt connects

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with the NW-trending Ougarta belt (Fig. 1A). Although both these belts result from the

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Variscan orogeny, it was suggested that Ougarta would have formed basically during the

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Permian contrary to the Anti-Atlas that would have mainly formed during the Bashkirian-

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Moscovian (Menchikoff, 1952; Fabre, 1971, 1976, 2005; Haddoum et al., 2001; Michard

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et al., 2008, 2010). Therefore, the area where these belts connect is potentially fitted for

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studying interference patterns of two sets of folds with different strike and age. In fact, the

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studied area exposes surprising structures such as the croissant- or boomerang-shaped

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Tijekht anticline, a surprising structure when seen in satellite view via Google earth. We

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propose in the following their interpretation in terms of thick-skinned inversion tectonics

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with superimposed folding events. Geological setting and stratigraphical

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outline

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The Anti-Atlas and Ougarta Paleozoic fold belts extend on the northern and north-eastern

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border of the West African Craton (WAC; Fig. 1A; Hollard et al., 1985; Ennih and Liégeois,

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2008), whose western border is made up by the Mauritanides (Sougy, 1962; Villeneuve et al.,

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2006; Michard et al., 2010). Their basement crops out in numerous faulted antiforms or inliers

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(“boutonnières”), which constitute as many opportunities to observe the Neoproterozoic Pan-

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African Belt which formed between ca. 700-640 Ma (Caby, 2003; Gasquet et al., 2008; Blein

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ACCEPTED MANUSCRIPT et al., 2014; Triantafyllou et al., 2015, and references therein). The Anti-Atlas inliers (Fig. 1B)

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south of the Anti-Atlas Major Fault (AAMF; Choubert, 1947) expose Paleoproterozoic

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terranes overlain by deformed and metamorphic deposits from the former WAC platform,

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recently dated from the Mesoproterozoic-Tonian (Ikenne et al., 2016). Along the AAMF

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itself, metaophiolites and oceanic arc units from the Pan-African suture zone dated at ca. 760-

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700 Ma crop out in the Siroua and Bou Azzer inliers. Northeast of the AAMF, namely in the

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Saghro and Ougnat massifs, only the youngest, 630-610 Ma-old (Liégeois et al., 2006; Abati

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et al., 2010, 2012) and lowermost-grade metamorphic units crop out beneath the

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unconformable late Ediacaran volcanic and volcaniclastic formations of the Ouarzazate

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Group. The latter group surrounds all the Anti-Atlas inliers, although with a strongly uneven

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thickness (Soulaimani et al., 2014), and accumulated between 575-550 Ma, being coeval with

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numerous HKCA granitoid intrusions (Gasquet et al., 2008; Blein et al., 2014).

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The overlying Paleozoic sequence (Michard et al., 2008, and references therein) begins

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with the lowermost Cambrian in the Western Anti-Atlas, but not before the late Early

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Cambrian sensu Destombes et al. (1985) or early Middle Cambrian, sensu Geyer & Landing

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(1995) in most of the Eastern Anti-Atlas (Fig. 2A), and not before the Middle Cambrian in the

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northern flank of the Ougnat Massif (Destombes & Hollard, 1986). This results from the

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activity of synsedimentary ENE-trending normal faults, also responsible for alkaline basalt

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outpours during the Early and Middle Cambrian (Raddi et al., 2007; Soulaimani et al., 2014).

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The Ordovician-Silurian period is characterized by the rather monotonous sandy to

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argillaceous deposits of the Saharan platform where the main perturbations occurred during

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the end-Ordovician glacial events (Destombes et al., 1985; Clerc et al., 2013; Ghienne et al.,

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2014, and references therein). The Middle-Upper Devonian deposits show dramatic thickness

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and facies variations (Fig. 2A, B) illustrating the coeval disintegration of the northern Saharan

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platform (Wendt, 1985; Baidder et al., 2008; Ouanaimi & Lazreq, 2008). Two subsiding

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ACCEPTED MANUSCRIPT basins formed at that time south of the Saghro-Ougnat and Erfoud high, namely the Maider

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and the South Tafilalt basins, separated by a paleogeographic high labeled the Ougnat-Ouzina

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Axis (Fig. 3). This occurred through synsedimentary normal faulting (Baidder et al., 2008)

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arguably due to back-arc extension in the foreland of the Rheic subduction (Michard et al.,

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2010) or to the effect of the Rheic subduction slab-pull assuming it occurred along the

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northwestern flank of the ocean (Gutiérrez-Alonso et al., 2008; Frizon de Lamotte et al.,

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2013).

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The youngest terms of the folded sequence are late Visean in age (Destombes & Hollard,

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1986). Post-folding, molasse-type subaerial deposits, Bashkirian and Pennsylvanianin age

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(from ca. 320 to 300 Ma; Fabre, 1976, 2005; Cavaroc et al., 1976) are preserved in the

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Tindouf cratonic basin south of the Anti-Atlas. In contrast, marine deposits accumulated up to

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the late Moscovian (ca. 305 Ma) in the Bechar-Abadla Basin to the east of the Ougarta belt.

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The Abadla basin received subaerial, red beds deposits during the late Westphalian-Autunian

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(Fabre, 1976, 2005; Bouabdallah et al., 1998), suggesting diachronic folding of the western

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Anti-Atlas and eastern Anti-Atlas-Ougarta belts.

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The Anti-Atlas Paleozoic fold belt is intruded by numerous dykes and sills of the Central

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Atlantic Magmatic Province, dated by place at 200-195 Ma (Hailwood & Mitchell, 1971;

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Hollard, 1973; Sebai et al., 1991; Derder et al., 2001; Youbi et al., 2003; Chabou et al., 2007;

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Verati et al., 2007), but no surface outpours or coeval sediments have been preserved except

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at the northern fringe of the central Anti-Atlas on top of the Abadla Basin (Fabre, 2005). The

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Paleozoic fold belt is surrounded by the unconformable, weakly faulted and tilted Cretaceous-

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Neogene deposits of the Saharan plateaus or hamadas (Draa and Guir Hamada, Kem Kem

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plateaus; Zouhri et al., 2008) to the south and east, and by those of the discontinuous, shallow

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sub-Atlas basins (Souss and Ouarzazate basins; Frizon de Lamotte et al., 2008) to the north.

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3. Methods The present work is mainly based on our detail mapping of the area (L.B., A.F.) covering

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the Al Atrous, Irara, Marzouga, Mfis and Taouz sheets of the Geological map of Morocco

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1:50,000, on the preparation (A.M.) of the corresponding explanatory notices (Benharref et

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al., 2014a-c; Alvaro et al., 2014a, b), and on several common field trips. The methods used

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besides of mapping are structural observations and measurements (bedding and fault planes,

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axes of minor folds, etc.) and analysis of satellite imagery, which is particularly informative in

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these arid regions. The thermal conditions that prevailed during deformation are defined

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through the observation of the structural features at the outcrop scale (bedding, joint systems,

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and locally spaced cleavage) or at the optical microscope scale (thin sections), and illite

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cristallinity measurements from the literature (Ruiz et al., 2008).

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4. Structure of the Southern Tafilalt-Maider area

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General structure and fault pattern

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The South Tafilalt-Maider area comprises five structural domains (Fig. 3). The

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northernmost one corresponds to the Ougnat Massif-Erfoud Anticlinorium structural axis that

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forms the eastern continuation of the Saghro Massif. This domain was a paleogeographic high

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during the Cambrian and again during the Devonian-Carboniferous (see above section). In

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particular, the Erfoud anticlinorium expose condensed Devonian formations typical for a

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pelagic high (Hollard, 1967, 1974; Wendt, 1985, 1988; Wendt and Belka, 1991; Baidder et

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al., 2008). The Ougnat-Erfoud domain is characterized by dominantly E-W structures. The

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Bouadil area south of the Ougnat Massif shows a mosaic of tilted basement blocks associated

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with dominantly NE- and SE-trending folds in the Paleozoic cover series (Raddi et al., 2007).

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In the Erfoud anticlinorium the Paleozoic succession also overlies Precambrian rocks in the

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north (Gour Brikat and Ras el Kahla tiny massifs; Fig. 1B). The southern boundary of the

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Ougnat-Erfoud structural domain is made up by globally E-trending faults such as the North

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and South Mecissi Faults and the Erfoud Fault.

The southernmost domain of the studied area is labeled hereafter the Kem-Kem Domain.

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The Paleozoic terranes there are widely hidden beneath the Hamada Cretaceous-Neogene

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formations that have been preserved due to a set of normal faults along the northern boundary

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of the domain. However, the structural pattern is well-defined by large SSE-trending folds

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such as the Ouzina and Aroudane anticlines. The Kem Kem Domain looks like the direct

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continuation of the Ougarta Belt (Fig. 1A; see Discussion section). This domain ends abruptly

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in the north when crosscut by the Oumjerane-Taouz Fault (OJTF). The latter is a complex

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fault zone extending over 250 km up to Zagora in the Central Anti-Atlas (Fig. 1B; (Baidder et

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al., 2008)) where it would connect with the Anti-Atlas Major Fault.

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The structural domains between the Ougnat-Erfoud and Kem Kem domains are threefold:

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two basinal domains, i.e. the Maider and South Tafilalt basins, respectively, and the

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intervening Ougnat-Ouzina Axis. The broadly quadrangular, synformal Maider Basin

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contains poorly deformed, relatively thick (~3000 m) Devonian-Lower Carboniferous

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deposits (Figs. 1B, 3). The northern border of the basin corresponds to the North and South

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Mecissi faults (NMF, SMF), and its southern border to the Oumjerane-Taouz Fault (OJTF). In

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the east the basin is bounded by the East Maider Fault (EMF) that connects with the Mecissi

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faults around the J. Signit through a system of curved faults, including the East Signit Fault

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(ESF). In contrast, the western border of the downwarped basin consists of a simple flexure

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zone. Scattered Middle Devonian reef mounds underline the borders of the adjoining platform

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areas, i.e. the Kem Kem Domain and the Ougnat-Ouzina Axis (Kaufmann, 1998).

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ACCEPTED MANUSCRIPT The South-Tafilalt Basin compares with the Maider basin, but it was more severely

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deformed than its western counterpart. The northern and southern borders of the South

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Tafilalt Basin are the same fault zones as that of the Maider Basin, whereas its western limit is

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a system of NW-trending faults, including the Oued Ziz Fault (OZF), mostly hidden beneath

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the Oued Ziz alluvium, and its northern branch that forms the Taklimt Fault Zone (TFZ). The

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Middle-Upper Devonian series crop out in two anticlines in the southeastern corner of the

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basin, namely the Mfis and Znaigui anticlines. The folded Lower Carboniferous beds occupy

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the wide Marzouga synclinorium. The famous Emsian-Givetian Hamar Laghdad mud mounds

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(Montenat et al., 1996; Mounji et al., 1998; Aitken et al., 2002; Cavalazzi et al., 2007; Franchi

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et al., 2014, 2015) are located on the northern slope of the South Tafilalt basin. The Lower

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Carboniferous series continue eastward beneath the Guir Hamada, then beneath the Upper

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Carboniferous-Permian deposits of the Bechar-Abadla area before outcropping again in the

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Saoura valley (Fabre, 1976, 2005). In other words, the subsiding basin is widely open to the

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east.

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The Ougnat-Ouzina Axis (“anticlinorium de Taouz” in Destombes, 2006a, b) is a large

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NNW-trending strip that exposes dominantly Cambrian and Ordovician terranes with

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subordinate Silurian and Devonian terranes preserved in narrow synclines. This uplifted axis

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separates the Maider Basin in the west from the South-Tafilalt Basin in the east (Fig. 3). The

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thick basinal infill contrasts with the thin coeval deposits of the intervening domain (Fig. 2),

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which derives from a paleogeographic high (Korn et al., 2000; Lubeseder et al., 2009). So, the

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EMF and OZF faults that bound the Ougnat-Ouzina Axis derive from the former paleofaults

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on both sides of the Devonian high. A number of secondary faults subdivide the domain into

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smaller units described in the following sub-sections.

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4.2.

The Kem Kem, Ougarta-type domain 9

ACCEPTED MANUSCRIPT The only outcrops of Precambrian and Early Cambrian beds in the whole Tafilalt area

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occur in the core of the Tazoult n’Ouzina anticlinal vault (Fig. 4A, B). This very open

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anticlinal structure is abruptly bounded northward by the OJTF whereas its axis plunges

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gently southeastward and forms (after a small dextral offset beneath the Oued Ziz alluvium)

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the core of the SE-trending, open Ouzina anticline. The Early Cambrian sandstones overlie

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directly the Precambrian brittle basement, so as the deepest potential décollement level

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corresponds to the Middle Cambrian Schistes à Paradoxides. However, the main décollement

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occurs in the Lower Ordovician (Fezouata and Tachilla Fms.; Fig. 2) between the Cambrian

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Tabanit sandstones and the quartzites and pelites of the Middle and Upper Ordovician

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formations.

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About 20 km further in the east, the Aroudane-J. Zorg anticline is quite similar in

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geometry and direction to the Tazoult n’Ouzina-Ouzina anticline. However, the axial

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culmination of the eastern fold is well-preserved (Cambrian massif of J. Zorg; Fig. 3) as the

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OJTF cuts the fold north of it, then offering a natural cross-section of the Ordovician envelope

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(J. Aroudane; Fig. 4C). Going again some 25 km to the east, another SE-trending anticline can

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be seen in the Silurian-Devonian formations at the foot of the Hamada (Oued Nebech area).

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Therefore, the Kem Kem structural domain is characterized by SSE- to SE-trending, quasi

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cylindrical open folds with 20-25 km wavelength, hardly detached from their Precambrian

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basement. This is typically the structure of the Ougarta belt (Donzeau, 1972, 1983; Zazoun,

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2001; Haddoum et al., 2001; Haddoum, 2009), whose northernmost folds are visible on the

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south border of the Kem Kem and Daoura Cretaceous-Neogene plateaus (Guir Hamada s.l.),

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i.e. at about 80 km south-southeast of Taouz (Fig. 1A).

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4.3.

Cambrian-cored folds of the Ougnat-Ouzina Axis

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ACCEPTED MANUSCRIPT The Tawjit n’Tibirene unit belongs to the group of tilted blocks described by Raddi et al.

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(2007) south of the Ougnat Massif. This unit is a large, almost monoclinal Cambrian slab that

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dips gently southeastward and forms the northeastern “root” of the Ougnat-Ouzina Axis.

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Three Cambrian-cored folded units occur in the Ougnat-Ouzina Axis itself, from north to

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south the J. Taklimt, J. Renneg and J. Tijekht units (Fig. 3). The surface of the Cambrian

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exposures increases from the Taklimt to the Tijekht units, which suggests a southward

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shallowing of the basement in the Ougnat-Ouzina Axis.

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The Taklimt unit in the northwest part of the Ougnat-Ouzina Axis is a good example of

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asymmetric, sub-cylindrical fold developed in correspondence with a deep fault zone that

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connects further in the SE with the Oued Ziz Fault (Figs. 5A, 3). This NW-trending fold is

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well designed by the First Bani quartzites that display box fold geometry next to its

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southeastern pericline. The southwestern limb of the fold is steeply dipping (Fig. 5B) in

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contrast with the northeastern. Taking into account the low temperature conditions of folding

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(see below, Discussion) and the brittle behavior of the basement, a dense set of faults must be

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hypothesized beneath the Middle Cambrian anticline within the basement and the overlying

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Lower Cambrian sandstones (Fig. 5C). Another branch of the Taklimt fault zone (TFZ) occurs

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along the NE limb of the fold, which separates the Taklimt unit from a foundered block

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transitional between the Ougnat-Ouzina Axis and the South Tafilalt Basin. A group of E-W

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open folds (including the Amelane and Mech Irdane synclines) are seen on this transitional

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block and reveals a dextral throw along the TFZ, coeval with what can be regarded as the

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main folding phase (“D1”; see sect. 5). Remarkably, the Taklimt fold is affected by a

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transverse fold whose axis plunges southward (Fig. 5A). This folding event suggests a minor,

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and probably late compressional event (“D2”) almost normal to the main one.

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ACCEPTED MANUSCRIPT The Renneg anticline is located along the opposite margin of the Ougnat-Ouzina Axis

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(Fig. 3). This structure is widely overlain by sandy deposits, which hampers its detail analysis.

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However, the bean-shaped horizontal section of its 8 km-long Cambrian core reveals the

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curvature of its axis. The direction of the Renneg axis is about N120E at its eastern pericline,

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but tends to parallel the EMF at its western pericline, suggesting a dextral throw along the

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fault.

The croissant- or boomerang-like J. Tijekht anticline is the southernmost, and most

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surprising Cambrian structure of the Ougnat-Ouzina Axis (Fig. 6A).

This structure is

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bounded in the east and south by two sinistral fault zones connected to the OJTF, i.e. the

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ENE-trending Tizi n’Ressas fault (TRF) and the latitudinal South Tijekht and Oumjerane-

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Taouz faults (STF and OJTF), respectively (Fig. 3). The deepest outcropping beds of the

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Tijekht anticline belong to the Schistes à Paradoxides Fm and their competent carapace is

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made up of Tabanit sandstones (Destombes & Hollard, 1986), the dip of which remains quite

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shallow everywhere (Fig. 6A, B). The broadly semicircular crest is in fact composed by two

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distinct parts separated by a reverse fault associated with two NNE-trending folds, suggesting

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a late, approximately E-W compression (cf. J. Taklimt “D2”). The eastern corner of the

279

Tijekht croissant broadly parallels the two transverse folds and can be associated with the

280

same compressional event. Remarkably, the system of fractures that affects both the eastern

281

and western corners of the croissant is a homogeneous N35-N70 system of steeply dipping

282

open faults mostly mineralized in barite (Fig. 6A). Thus these fractures record an ultimate

283

tectonic event (“D3”) with a broadly NE-directed horizontal compression. In addition to the

284

main system of fractures, the conical periclines are truncated by transverse normal faults.

AC C

EP

TE D

M AN U

270

285

West of the TRF and north of the STF-OJTF boundary faults, the croissant-shaped

286

Cambrian core is surrounded and overlain by Ordovician formations whose geometry is much 12

ACCEPTED MANUSCRIPT simpler. Above the thick, lower Ordovician pelites (Fezouata and Tachilla Fms.) that drape

288

the Tabanit irregular vault, the competent formations of the First Bani, Ktaoua and Second

289

Bani are organized in two sets of relatively tight folds slightly fanned with respect to the NW-

290

SE direction (Figs. 3, 6A&). Their mean direction is precisely that of the west corner of the

291

Cambrian massif. This implies that the main folding event responsible for the structure of the

292

Tijekht unit corresponds to a NE-trending compression, also found in the J. Taklimt (“D1”

293

event) and J. Renneg (eastern part). In contrast, the east corner of the Tijekht Cambrian massif

294

would result from the transverse “D2” event evidenced in the J. Taklimt (see Discussion

295

section).

296

4.4.

M AN U

SC

RI PT

287

Ordovician-cored folds of the Ougnat-Ouzina Axis

In this section we consider three antiformal units (Fig. 3), from north to south: i) the Bou

298

Mayz anticline immediately south of the Taklimt and Amelane-Mech Irdane units studied

299

above; ii) the large Shayb Arras anticline and its second order folds, and iii) the J. Tadaout

300

system in the southeastern most part of the Ougnat-Ouzina Axis, immediately north of the

301

OJTF. These units are cored by Ordovician terranes and separated from each other by narrow

302

Devonian-Carboniferous synclines, namely the Ottara and Amessoui synclines north and

303

south of the Shayb Arras anticline, respectively. As the Cambrian does not crop out in these

304

anticlines we infer they are built over low basement blocks with respect to the Tijekht and

305

Renneg Cambrian-cored structures. In other words, the basement is higher in the west of the

306

Ougnat-Ouzina Axis than in the east, which is in fact inherited directly from the Devonian

307

paleogeography (Korn et al., 2000; Lubeseder et al., 2009).

AC C

EP

TE D

297

308

The Bou Mayz anticline globally consists of an E-trending, 20 km-long cylindrical fold

309

made up of Upper Ordovician formations and bounded southward by the Ottara syncline (Fig.

13

ACCEPTED MANUSCRIPT 3). However, the western pericline of the fold is particularly interesting as it displays a clear

311

interference pattern (Fig. 7A). The main fold is crosscut here by transverse folds trending

312

NNE-SSW with crest lines strongly curved in their vertical axial plane. These secondary folds

313

are reminiscent of the “D2” fold observed in the J. Taklimt at a short distance further north.

RI PT

310

The crest of the Bou Mayz anticline describes a dextral bayonet at about half its length, in

315

the area where the Oued Rheris crosses the fold. Some 6 km further in the east, the eastern

316

pericline is twisted dextrally to a SE direction close to the Oued Ziz dextral strike-slip fault

317

(OZF). Most of the NE-trending fractures here are mineralized in barite, which compares with

318

the fracture systems of the Tijekht (see above) and Shayb Arras anticlines.

M AN U

SC

314

The Shayb Arras anticline is remarkable in two respects, i) its overall axis is sigmoidal,

320

and ii) its core of Middle Ordovician formations displays a brachyanticlinal shape contrasting

321

with the elongated shape of the Upper Ordovician-Devonian envelope (Fig. 8A). The

322

sigmoidal shape of the whole structure is particularly clear in the Silurian-Devonian eastern

323

pericline. There, the curvature of the axis occurs through a complex pattern of strike-slip,

324

normal or reverse faults suggesting brittle deformation of a previously more rectilinear

325

cylindrical fold (Fig. 8A, D). This forced curvature is consistent with the dextral throw along

326

the OZF, already documented further in the north (J. Taklimt and Bou Mayz region; Fig. 3).

327

The western curvature is less visible and occurs along a sinistral strike-slip fault that follows

328

the southwest border of the Mech Agraou plateau, surrounds the Amessoui western pericline

329

and then follows eastward the southern border of the syncline, thus being labeled Amessoui-

330

Mech Agrou fault (AMF; Fig. 8E). The AMF basically appears at the surface as a corridor of

331

en echelon folds or asymmetric shear folds in the Devonian formations, but likely corresponds

332

to a basement fault at depth.

AC C

EP

TE D

319

14

ACCEPTED MANUSCRIPT The central brachyanticline of the Shayb Arras structure is defined by the First Bani

334

quartzites that show a box-type profile formed through buckling above the incompetent

335

Lower Ordovician pelites. The geometry of the Upper Ordovician competent formations

336

(Tiouririne sandstones, Second Bani) broadly mimics that of the First Bani, but second order

337

flexural folds develop in these beds outside the core area (Fig. 8A). Second order kink folds

338

are also observed in the Middle Devonian limestones of the northeastern limb of the major

339

fold (Fig. 8B). Layer-parallel shear is documented by minor folds at varied places (Fig. 8C).

340

The entire anticline is crosscut by a set of vertical faults directed NE to ENE and frequently

341

mineralized in barite. The walls of these veins bear conspicuous horizontal striations with

342

sinistral kinematic indicators (Fig. 8F). All these observations document a flexural-slip

343

mechanism of folding that evolved toward a more brittle style of deformation. The shortening

344

direction would have rotated from N-S to NE-SW in the meanwhile.

M AN U

SC

RI PT

333

Similar to the Amessoui syncline, the Ottara syncline is also bordered by a sinistral fault

346

corridor broadly parallel to its axis. At the western pericline (Fig. 9), the fault is located south

347

of the fold and curves northwest-ward as the syncline axis. Several transverse faults are also

348

observed in the pericline, that likely result from the complete or partial inversion of the

349

normal paleofaults that were active during the Middle-Upper Devonian (Lubeseder et al.,

350

2010).

AC C

EP

TE D

345

351

The Tadaout massif is located in the trapezoidal block bounded by the OZF and OJTF

352

main faults in the east and south, and the AMF and TRF subsidiary faults in the north and

353

west, respectively (Fig. 3). In other words, the Tadaout massif occupies the very southeast

354

corner of the Ougnat-Ouzina Axis, and this probably accounts for the great structural

355

complexity of this kind of “Gordian knot” (Fig. 10). The rock material involved in the massif

356

spans from the Lower Ordovician Fezouata pelites to the Lower Carboniferous in the faulted 15

ACCEPTED MANUSCRIPT J. Mraier syncline, which suggests a downwarped basement with respect to the adjoining J.

358

Tijekht massif and Kem Kem domain. The internal structure of the Tadaout massif can be

359

untangled by distinguishing two sets of superimposed folds, broadly E-W and N-S,

360

respectively. The major and earlier folds appear to be the E-W directed, better said N100 in

361

the west to N120 in the east, suggesting a dextral twist comparable with that observed in the

362

Shayb Arras unit. In contrast, the N30 to N160-trending folds appear to postdate the E-W

363

ones as they are linked to transverse faults crosscutting the latter folds. These faults are the

364

Tadaout Central fault (TCF) in the middle of the massif and the Bou Hmid and Tizi n’Ressas

365

faults (BMF and TRF) in the west. The BMF fault is rectilinear and parallel to the TRF in the

366

southwest, but it curves in the north around the J. Bou Hmid monocline, which is the substrate

367

of the Mraier syncline, and finally ends against the TCF (Fig. 10). Thus this very peculiar

368

fault seems to detach the Mraier-Bou Hmid unit from the south part of the Tadaout massif and

369

carry it further in the north like a drawer between two N20-striking ramp faults. Along the

370

west border of the tectonic drawer and at its front, the Silurian-Lower Devonian limestones

371

show numerous minor folds recording the displacement of the Mraier-Bou Hmid unit, which

372

however was probably limited to less than 1 km. The N-S compression of the Tadaout block

373

against the Cambrian-Ordovician Kem Kem domain in the south is attested by the occurrence

374

of hectometric lenses of verticalized beds and the coexistence of dextral and sinistral minor

375

structures in the OJTF (Fig. 11).

SC

M AN U

TE D

EP

AC C

376

RI PT

357

4.5.

Structure of the Maider and South Tafilalt basins

377

In this section we briefly consider the folds that affect the Devonian-Lower

378

Carboniferous formations of the two basinal areas on both sides of the Ougnat-Ouzina Axis,

379

i.e. the Maider and South-Tafilalt Basin (Fig. 3). The overall outline of these basins has been

380

defined above (sect. 4.1). 16

ACCEPTED MANUSCRIPT The quadrangular Maider Basin is poorly deformed, except along its borders. The

382

Ordovician-Lower Devonian series crop out as E- to ENE-trending anticlines in the south

383

(Msiouda). Submeridian fold axes are observed in the NE corner of the quadrangle, where

384

they would record E-W compression against the Ougnat-Ouzina Axis. A NNW-plunging

385

minor fold affects the Lower and Middle Devonian formations of the east border of the basin,

386

consistent with a dextral throw along this inverted paleofault zone. Lastly, a poorly marked

387

anticline occurs in the southeastern part of the basin, resulting in a wide exposure area of the

388

lowest Upper Devonian series there. The remarkable sinusoidal shape of the Fezzou Lower

389

Carboniferous syncline cannot be easily accounted for by fold interference, and would rather

390

result from the adaptation of the sedimentary infill to the inversion of the surrounding or

391

underlying paleofaults during a moderate NW-SE to NE-SW compression. In particular, the

392

NE-striking, NW-dipping Fezzou paleofault documented by the Devonian stratigraphy

393

(Baidder et al., 2008) would have controlled the NE trend of the syncline axis east of Fezzou

394

village.

TE D

M AN U

SC

RI PT

381

The South-Tafilalt Basin is much more deformed than its western homologous. The most

396

complex structures appear in the Znaigui and Mfis anticlines in the southwest corner of the

397

basin bounded by the OJTF and OZF fault zones (Fig. 13A). Both anticlines show exposures

398

of Middle-Upper Devonian competent formations. They are crosscut by ENE-trending

399

sinistral faults. The Mfis anticline displays a brachyanticlinal geometry, which could

400

corresponds to the interference of a submeridian “D2” fold superimposed on an latitudinal

401

“D1” fold. The core of the fault is crosscut by a complex set of open faults mineralized in

402

barite (Fig. 13B), and intruded by several dolerite bodies of probable Triassic-Liassic age.

403

The Marzouga synclinorium in the central part of the basin displays WNW-ESE directed fold

404

axis, broadly parallel with those of the Erfoud anticlinorium in the Widane Chebbi area. Some

405

hectometric second order folds appear in the major hinges (e.g. Hassi Merdani area), and the

AC C

EP

395

17

ACCEPTED MANUSCRIPT dip of bedding may reach about 60° in the highest stratigraphic levels within the deeper part

407

of the synclinorium, east of Hamou Rhanem. This attests for the importance of the

408

submeridian shortening of the sedimentary basin between the northern (Erfoud) and southern

409

(Nebech) uplifted blocks. The weak inflexion of the fold axes toward a NW direction next to

410

the OZF is consistent with a dextral movement along this fault zone.

411

5. Discussion 5.1.

Folds interferences and involvement of the faulted basement

SC

412

RI PT

406

The structural data presented above are gathered together in a synthetic map (Fig. 14). At

414

first glance, this map illustrates the varied, interfering directions of fold axes and their close

415

relationships with the regional fault array:

416

-

M AN U

413

in the southernmost Kem Kem domain, folds and faults are regularly oriented NNWSSE, which is the main Ougarta direction (Menchikoff, 1952; Donzeau, 1972, 1974,

418

1983; Zazoun, 2001; Haddoum et al., 2001); in the central and northern domains, i.e.

419

the Ougnat-Ouzina Axis, the South Tafilalt Basin, and the south border of the Ougnat

420

Massifand Erfoud Anticlinorium, fold axes are dominantly directed WNW-ESE to E-

421

W, which is the direction of the Hercynian structures in the eastern Anti-Atlas and

AC C

EP

TE D

417

adjoining Meseta units (Fig. 1B); the main faults strike either E-W to ENE or NW-SE,

422

which are the dominant fault directions in the eastern Anti-Atlas and Ougarta belts,

423

respectively;

424 425

-

most fold axes are sigmoidal, and the curvature of their periclines give evidence of

426

strike-slip displacements along several fault zones; dextral movements are particularly

427

documented along the EMF and OZF faults that bound the Ougnat-Ouzina Axis, as

428

well as along the TFZ branch of the latter; sinistral displacements are documented 18

ACCEPTED MANUSCRIPT 429

along dominantly E-W to ENE-striking faults or strike-slip corridors such as the STF,

430

AMF, SOF structures (from south to north within the Ougnat-Ouzina Axis);

431

-

clear fold interferences occur in the Taklimt and Bou Mayz anticlines in the north of the Ougnat-Ouzina Axis, with the NW- to W-trending main folds “D1” deformed by

433

NNE- to N-S trending minor folds “D2”; the relative chronology of these folds is

434

discussed in the following section;

435

-

RI PT

432

folds at the southern part of the Ougnat-Ouzina Axis, next to the major OumjeraneTaouz fault (OJTF) show strong axis curvature and internal faulting; the Tijekht

437

anticline offers a croissant or boomerang shape in map view, and the adjoining

438

Tadaout anticlinal massif looks like a Gordian knot of intersecting folds and faults.

439

So, folding interferences and folds peculiar geometries appear to be controlled by an

440

array of intersecting faults in the basement allowing relative displacements of basement

441

blocks to occur. These basement faults correspond to inverted synsedimentary faults, mostly

442

Devonian paleofaults (Baidder et al., 2008) as exemplified in particular by the EMF, OZF and

443

OJTF faults (Fig. 15; see also sect. 2 and 4). Hence, we deal with a thick-skinned inversion

444

tectonics as observed immediately in the north around the Ougnat inlier (Raddi et al., 2007)

445

and further in the west in the central Anti-Atlas (Faik et al., 2001; Burkhard et al., 2006). The

446

synthetic profile here proposed (Fig. 16) makes visible the involvement of the brittle

447

basement in the first order folds of the cover. Folding of the sedimentary cover above the

448

moving mosaic of basement blocks was permitted because of the occurrence of ductile, pelitic

449

or argillaceous formations (Schistes à Paradoxides, Fezouata-Tachilla pelites, Silurian shales

450

and upper Emsian marls). The Lower Cambrian sandstones remained globally stuck onto the

451

basement, in the absence of thick “lie-de-vin” pelites and layered limestones at the bottom of

452

the sequence, which contrasts with the Western Anti-Atlas setting (Helg et al., 2004;

AC C

EP

TE D

M AN U

SC

436

19

ACCEPTED MANUSCRIPT 453

Burkhard et al., 2006). Folds are very open in the Cambrian Tabanit sandstones, whereas they

454

become tighter in the overlying Ordovician and Devonian formations.

In this profile, the dip of the strata has been extrapolated at depth admitting a flexural-slip

456

mechanism of folding, which is well-documented by the field observations at every

457

stratigraphic level and consistent with the low-temperature conditions of the Variscan

458

deformation (see next section). The brittle behavior of the basement in such conditions is

459

illustrated in the Ougnat Massif (Raddi et al., 2007) and Erfoud anticlinorium in the north, as

460

well as in the Ediacaran outcrops beneath the Tazoult n’Ouzina vault in the south. The Dip of

461

the faults at depth remains speculative. Unpublished seismic profiles acquired by the Office

462

National de Recherche et d’Exploitation Pétrolière (ONAREP, now renamed ONHYM,

463

Office National des Hydrocarbures et des Mines) in the Erfoud-Rissani basin have been

464

tentatively interpreted in the last couple of years (Baidder, 2007; Toto et al., 2008; Robert-

465

Charrue and Burkhard, 2008), but resulted contradictory due to the poor quality of these

466

ancient 2D-seismic lines. Here the style of faulting is inspired from the Laramide examples

467

(Mitra and Mount, 1998) on the one hand (Tijekht and Shayb Arras anticlines, Erfoud

468

anticlinorium), and on the other hand from the flower geometry (Harding, 1985) where

469

vertical throw is minimum (e.g. Mech Agrou-Amessoui strike-slip fault).

AC C

EP

TE D

M AN U

SC

RI PT

455

470

Fold orientation is mostly oblique to the major basement faults (Fig. 14), suggesting the

471

regional stress was oblique to these faults during at least part of the Variscan orogeny.

472

However, along some of the inverted paleofaults the development of multiple shear planes

473

may result in pseudo continuous deformation of the basement at the vertical of a fold in the

474

cover. This was described in the case of the J. Angad anticline in the Bou Adil area south of

475

the Ougnat Massif (Raddi et al., 2007), and seems also appropriate to the J. Taklimt case in

476

the Ougnat-Ouzina Axis (Fig. 5). 20

ACCEPTED MANUSCRIPT 477

478

5.2.

Low temperature conditions of strain

In order to ascertain the above interpretation of the regional tectonic style, it is

480

appropriate specifying the physical conditions that prevailed in the rock material during

481

folding. This is particularly true to understand the formation of the Tijekht and Tadaout

482

croissant-shaped folds, seldom described in the literature. Besides of crescent folds associated

483

occasionally with diapirism (Jackson et al., 1990), crescent fold pattern caused by flattening

484

and flexural flow folding have been described in the southernmost Altaids where the

485

sediments were unconsolidated and enriched in fluids during their deformation (Tian, 2013),

486

which is clearly not the case of the Cambrian and Ordovician strata of the Tafilalt area.

M AN U

SC

RI PT

479

In the South Tafilalt area, the P-T-fluid content conditions were different during folding

488

from top to bottom of the Paleozoic sedimentary pile. In the Lower Carboniferous formations,

489

rocks were buried at > 2 km depth (Fig. 2), perhaps ca. 5 km assuming ~3 km-thick eroded

490

deposits, and they were still rich in fluids when the Late CarboniferousEarly Permian folding

491

occurred. Therefore, and in the absence of any coeval magmatism, folding occurred at very

492

low temperature (150°C at a maximum) and pressure. Illite cristallinity measurements

493

confirm that these rocks remained in diagenetic conditions, i.e. at T< 200°C (Ruiz et al.,

494

2008). Contrary to Benharref et al. (2014) who suggest that the planar fabric observed in these

495

Carboniferous rocks is a metamorphic foliation, we consider that it corresponds generally to

496

the stratification plane enhanced by compaction and locally deformed around the calcareous

497

or cherty concretions (Fig. 17A, B). However, a true tectonic cleavage (Fig. 17C) is observed

498

south of Taouz in the olistolite-bearing deposits accumulated against the OJT fault during the

499

Tournaisian (Fig. 4C). This tectonic fabric is a vertical, spaced cleavage axial-planar to

AC C

EP

TE D

487

21

ACCEPTED MANUSCRIPT decametric folds and almost parallel to the adjoining fault. The asymmetry of these folds and

501

their steeply dipping axis (N70E, 50-60° ENE) indicate a sinistral throw during compression

502

of the Lower Carboniferous series against the Ordovician quartzites of J. Aroudane (Figs. 4,

503

13). The spaced cleavage developed there by pressure-solution at low temperature as the

504

Tournaisian sediments were still rich in fluids.

RI PT

500

When folding began, burial of the lowermost Paleozoic beds exceeded that of the Lower

506

Carboniferous by ca. 3 km, which is the mean thickness of the Cambrian-Devonian series in

507

the area (Fig. 2). The expected temperature was likely close to 200°C assuming a 25°C/km

508

geotherm, which is typical for continental basins with thick infill (Allen & Allen, 1990). The

509

illite cristallinity indexes measured by Ruiz et al. (2008) in Devonian, Silurian and Ordovician

510

samples from the area indeed indicate diagenetic to anchizonal evolution, whereas epizonal

511

conditions are not observed here contrary to the western and central Anti-Atlas regions. In

512

other words, T remained close to 200°C in most of the Tafilalt-Maider area. This is consistent

513

with the sedimentary fabric observed in the Middle Cambrian formations (e.g. Tijekht

514

anticline; Fig. 17D).

M AN U

TE D

Superposed folding events or fault control during a single deformation

EP

5.3.

AC C

515

SC

505

516

This classical problem (e.g. Marshak, 2000; Carciumaru and Ortega, 2008) was addressed

517

in the western-central Anti-Atlas by Faik et al. (2001) and by Martin Burkhard and his alumni

518

(Caritg et al., 2004; Helg et al., 2004; Burkhard et al., 2006) with divergent conclusions. In

519

agreement with Soulaimani (1998), Faik et al. (2001) suggested that the fold interferences of

520

the Tata area were controlled by the inverted paleofaults orientation without any superposed

521

events of differently oriented regional compression. In contrast, Burkhard’s school favored a

522

combination of paleofault control and superposed compression events, oriented firstly south-

22

ACCEPTED MANUSCRIPT 523

eastward, then southward. We argue that such a combination of tectonic events fits the

524

Taflilat study case, although with different orientations of compression with respect to

525

Western Anti-Atlas.

First of all, rotation of the direction of compression is clearly documented in the

527

Variscan belt of the Meseta-Atlas domain, from a dominant WNW-ESE trend during the

528

Bashkirian-Moscovian to a N-S trend during the Late Pennsylvanian-Early Permian (De

529

Koning, 1957; Ferrandini et al., 1987; Aït Brahim and Tahiri, 1996; Saidi et al., 2002; Saber

530

et al., 2007). . Indeed, a similar rotation of regional stress is observed as well in the Variscan

531

belt of Western Europe (Marques et al., 2002; Ribeiro et al., 2007; Gutiérrez-Alonso et al.,

532

2015).

M AN U

SC

RI PT

526

As the Meseta-Atlas domain was coupled with its metacratonic foreland along the SMF

534

(Fig. 1) from the Bashkirian onward, the Late Pennsylvanian-Early Permian rotation of

535

compression occurred also in the Anti-Atlas, and likely in the Ougarta belt further in the

536

south-east. The regional stress reorientation from NW-SE to N-S was described by Caritg et

537

al. (2004) in western-central Anti-Atlas, as reported above. Rotation of regional stress is also

538

reported in the Tineghir area from N-S to NNW-SSE during the same Late Carboniferous-

539

Early Permian span of time (Soualhine et al., 2003; Cerrina-Feroni et al., 2010). In the Eastern

540

High Atlas Tamlelt massif of the South-Meseta Zone immediately north of the Bechar Basin,

541

Variscan E-W folding and dextral shearing record a similar evolution of compressional trend

542

(Houari and Hoepffner, 2003).

AC C

EP

TE D

533

543

In the Ougarta-Ahnet belt, folding would have started shortly after the Stephanian-

544

Autunian (Haddoum et al., 2001), whose subaerial red beds deposits (Abadla lower and upper

545

formations; Fabre, 1976, 2005; Bouabdallah et al., 1998) are tilted along the western and

23

ACCEPTED MANUSCRIPT eastern sides of the belt (Reggane and Bechar Basin, respectively). However, some structural

547

observations suggest that a Visean deformation event occurred (Blès, 1969), which appears

548

consistent with K-Ar datings of <2µ mica fractions from three Ougarta samples at 378±17,

549

323±9 and 246±7 Ma (Bonhomme et al., 1996). Likewise, whole-rock K-Ar datings of

550

Ediacaran volcanics yielded 310 Ma and 264 Ma ages (Hamdidouche and Aït Ouali, 2009).

551

Thus a protracted Variscan evolution of the Ougarta belt must be considered rather than a

552

single Early Permian event. Lamali et al. (2013) even proposed the occurrence of a

553

Famennian-Tournaisian event, based on the paleomagnetic study of the magmatic complex of

554

the Precambrian-Cambrian inliers. This proposal is contradicted by the perfect continuity

555

between the Devonian, Tournaisian and Visean strata of the fold belt (Menchikoff, 1952;

556

Haddoum et al., 2001; Haddoum, 2009). So, we retain at least provisionally that Ougarta

557

deformation occurred during the Late Carboniferous-Early Permian, being coeval of the Anti-

558

Atlas folding.

TE D

M AN U

SC

RI PT

546

The rotation of regional stress directions is also documented in Ougarta, and fold

560

interferences are also described there (Collomb and Donzeau, 1974; Haddoum, 2009). The

561

main direction of shortening changes from NE-SW to E-W, which is interpreted either as the

562

result of superposed events (Collomb and Donzeau, 1974) or as a continuous reorientation

563

process (Zazoun, 2001). Anyway, the control of fold trends by NW and E-W striking

564

basement faults inherited from the Pan-African orogeny is generally acknowledged in the

565

literature and explain the dominant NW to NNW trend of the Paleozoic belt. The global

566

model that better accounts for this evolution evokes the impingement of the WAC nucleus

567

against the European Variscan belt in the north and the East Sahara metacraton (Ennih and

568

Liégeois, 2008) in the east, linked to a northward and anticlockwise rotational movement of

569

Africa during the Alleghanian-Variscan collision (Lefort, 1988; Lefort and Bensalmia, 1992).

AC C

EP

559

24

ACCEPTED MANUSCRIPT The interpretation of the Tafilalt-Maider structures must be approached within this

571

general framework as a combination of paleofault control of fold orientation and superposed

572

compressional events with different directions of compression. In particular, this general

573

scenario may account for the most complicated fold structures of the area, i.e. the croissant-

574

shaped Tijekht anticline and the curved and fractured Tadaout anticline (Fig. 18).

RI PT

570

The first stage of our qualitative model (Fig. 18A) delineates the synsedimentary normal

576

fault array active during the Middle-Upper Devonian to Early Carboniferous. Four uplifted

577

blocks are distinguished: two of them belong to the Kem Kem domain south of the OJTF

578

whereas the other two belong to the Ougnat-Ouzina Axis. The latter blocks are supposedly

579

crosscut by broadly N-S normal faults that would account for the eastward thickening of the

580

Devonian series and the associated debrites facies (Korn et al., 2000; Lubeseder et al., 2009)

581

and for the subsequent activation of strike-slip faults like the Tizi n’Ressas fault (TNR, Fig.

582

14).

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The earliest compressive event “D1” corresponds to the N-S shortening of the Meseta-

584

Anti-Atlas system reported above, and dated from the Bashkirian-Westphalian. At that stage

585

(Fig. 18B), the South Tafilalt-Maider area is deformed north of the OJTF and the latter fault is

586

partly inverted as a sinistral strike-slip zone. The Ougnat-Ouzina Axis becomes a mega shear

587

zone between the Oued Ziz (OZF) and East Signit-East Maider paleofaults (ESF, EMF),

588

partially inverted into dextral strike-slip faults. The E-W trending folds born at the beginning

589

of this “D1” event become sigmoidal. The basement of the Shayb Arras and Tijekht-Tadaout

590

anticlines tend to shorten through conjugate strike-slip faulting and intense shearing along the

591

inverted paleofaults.

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ACCEPTED MANUSCRIPT The Ougarta events are characterized by NE-SW to E-W compression dated as

593

Stephanian-Early Permian. The youngest “D2” event (Fig. 18C) is firstly responsible for the

594

development of NNW-trending, Ourgata-like folds south of the OJTF (e.g. Ouzina and

595

Aroudane-Zorg anticlines, Fig. 4). Second, north of the OJTF this event superimposed a

596

transverse shortening onto the earlier structures. The width of the Ougnat-Ouzina mega shear

597

zone lessens and the pre-existing folds are deformed by the development of transverse,

598

broadly N-trending secondary folds. This is exemplified in the north of the Ougnat-Ouzina

599

Axis by the abrupt kink of the J. Taklimt crest (Fig. 5) and the egg-box pattern at the western

600

pericline of the Bou Mayz anticline (Fig. 7). In the southern part of the Ougnat-Ouzina Axis

601

and adjacent South Tafilalt Basin, the effect of the “D2” shortening is twofold. First, the

602

sigmoidal shape of the Shayb Arras anticline and adjacent synclines is accentuated and

603

second, the geometry of the core of the anticlines is modified. At last, an axial culmination

604

deforms the crest of the earlier fold that becomes a brachyanticline in the core region (e.g.

605

Shayb Arras and Mfis anticlines). More significantly, early and secondary fold axes may

606

complicate the geometry as to form croissant- or boomerang-shaped anticlines, either

607

relatively simple (Tijekht anticline, Fig. 6) or deeply fractured (Tadaout anticline, Fig. 10).

608

The poles to bedding are strongly scattered at the scale of the whole region (Fig. 18D). At the

609

scale of individual anticlines, the distribution of the downdip lines of bedding makes visible

610

the contrast between the Kem-Kem Domain (Figs. 4A, D) with its simple, north-trending

611

folds, and the domains north of the OJTF (Figs. 5, 6, 8, 13), where interference of folding

612

episodes are best exposed.

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613

It is worth noting that deformation of the Paleozoic terranes did not stop at this stage

614

“D2”. The occurrence of a widespread system of barite veins oriented rather constantly in the

615

NE-SW quadrant in most of the studied anticlines suggests a “D3” stage of NW-SE extension

616

during which the fractures probably created by the previous “Ougarta event” opened and were 26

ACCEPTED MANUSCRIPT mineralized. This could be ascribed to the well-known Triassic-early Liassic rifting event

618

(Frizon de Lamotte et al., 2008, and references therein; Berrada et al., 2016). The advection of

619

the mineralizing solution may have been enhanced by the Late Triassic magmatic event by the

620

end of the rifting process as suggested by Kharis et al. (2011) for the Oumjerane veins hosted

621

in the Ordovician quartzites west of the Maider Basin.

622

6. Conclusion

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Sub-Saharan Morocco offers optimal conditions for studying fold geometry and folds and

624

faults relationships in the frame of a thick-skinned foreland belt, namely the Anti-Atlas

625

Paleozoic belt. The present work focused on the Eastern Anti-Atlas where the E-W trending

626

Anti-Atlas connects with the NW-trending Ougarta. Both belts formed during the Variscan

627

(Alleghanian-Hercynian) Late Carboniferous-Early Permian collision between Laurentia-

628

Avalonia and Gondwana, but they developed in distinct structural setting. The Anti-Atlas

629

formed at the expense of the northern margin of the WAC cratonic domain whereas Ougarta

630

developed at the expense of an elongated trough between the WAC and the East-Sahara

631

metacraton. Deformation was possibly slightly diachronic from west to east as sedimentation

632

changed from marine to subaerial during the Bashkirian-Westphalian transition in the west

633

and not before the late Moscovian in the east. All around the north-eastern border of the

634

WAC, the paleofault pattern was different from west to east, showing NE and E-W strikes in

635

the west and E-W to NW-SE strikes in the east.So, the South Tafilalt-Maider area appears as a

636

good example of inversion tectonics with a dual mechanism of fold interference by both fault

637

control of the basement-cored folds and superposed compressional events with different

638

compression trend. Probably the most curious result of this dual mechanism corresponds to a

639

large croissant- or boomerang-shaped Cambrian anticline easily observed in satellite imagery.

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27

ACCEPTED MANUSCRIPT 640

We consider the area as a valuable target for advanced structural research on selected

641

individual folds.

642

Acknowledgements

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We are greatly indebted to one of our reviewers, Dominique Frizon de Lamotte, for his

645

accurate and friendly criticism of the earlier version of this work. The Direction of Geology,

646

Ministry of Energy and Mines, Water and Environment, Rabat (Dr. Belkhedim) afforded us

647

logistic support for our conclusive field trip, April 2015.

648

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Sci. 116, 331–358.

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43

ACCEPTED MANUSCRIPT Fig. 1. The Anti-Atlas Paleozoic fold belt. A: Location at the front of the Variscan belt.

980

Dashed line: Front of the Variscan deformation. A-A: Anti-Atlas; BB: Bechar Basin; MAU:

981

Mauritanides; ME-AT: Meseta-Atlas Variscides ; OUG : Ougarta intracontinental belt ; RB:

982

Reggane Basin; SAF : South Atlas Fault; TB: Tindouf Basin; WAC: West African Craton. B:

983

Structural map of the Anti-Atlas after Soulaimani and Burkhard (2008) and Michard et al.

984

(2010), modified. At this scale, the Mesozoic-Cenozoic South Atlas Fault is distinct from the

985

Paleozoic South Meseta Fault, except locally (Tizi n’Test Fault, between the Ouzellarh and

986

Western High Atlas blocks). BA: Bou Azzer inlier; GB: Gour Brikat; OG: Ougnat Massif;

987

ZE: Zenaga inlier.

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Fig. 2.

990

Carboniferous series of the Ougnat-Ouzina high axis between the Maider and South Tafilalt

991

basins. B: Devonian-Carboniferous series of the South Tafilalt Basin east of the high axis.

992

Stratigraphic symbols, thicknesses and facies after the explanatory notices of the geological

993

maps, scale 1:50,000, sheets Al Atrous, Irara, Marzouga, Mfis and Tawz (Benharref et al., in

994

press).

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Generalized stratigraphic columns of the Tafilalt region. A: Cambrian-Early

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996

Fig. 3. Main structural domains of the South Tafilalt-East Maider area. Landsat image N30-

997

30-2000 interpreted after the geological map of Morocco, scale 1:200,000 (Destombes &

998

Hollard, 1986) and this work. See Fig. 1B for location. AMF: Amessoui-Mech Agrou Fault;

999

EF: Erfoud Fault; EMF: East Maider Fault; ESF: East Signit Fault; N/SMF: North/South

1000

Mecissi Faults; OJTF: Oumjerane-Taouz Fault; OZF: Oued Ziz Fault; STF: South Tijekht

1001

Fault; TRF: Tizi n’Ressas Fault.

44

ACCEPTED MANUSCRIPT 1002

Fig. 4. Cambrian-cored structures of the Kem Kem Domain. A: Interpreted Google earth

1004

image of the Tazoult n’Ouzina fold. Fig. 3 for location. Stratigraphic symbols as Fig. 2.

1005

Insert: statistical orientation of bedding (downdip lines, equal angle, lower hemisphere). B:

1006

View of Tazoult n’Ouzina from the Early-Middle Cambrian core of the fold (see location in

1007

A). C: Profile of the eastern limb of the J. Aroudane fold as seen from the Lower

1008

Carboniferous outcrops north of the OJTF. Notice the blocky facies (olistostrome) of the

1009

Tournaisian deposits suggesting synsedimentary activity of the fault. D: Stereographic

1010

projections of bedding of the J. Aroudane fold.

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Fig. 5. The J. Taklimt fold in the northeastern Ougnat-Ouzina Axis (Fig. 3 for location). - A:

1013

Interpreted Google earth image. Stratigraphic symbols as Fig. 2. Anticlinal axes symbols: full

1014

for the main folding phase, empty for the subsequent deformation (twist of the main fold

1015

axis). Insert: statistical orientation of bedding downdip lines (equal angle, lower hemisphere).

1016

– B: View of the southern crest of the First Bani box fold (see A for location). – C: Cross-

1017

section interpreted at depth admitting a flexural-slip folding mechanism. Notice the link of the

1018

anticline with a system of basement faults (Taklimt Fault Zone E and W, TFZ (E), TFZ (W) =

1019

inverted normal faults at the boundary of the South Tafilalt Basin).

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Fig. 6. The boomerang- or croissant-shaped Tijekht Cambrian anticline and surrounding

1022

Ordovician folds. – A: Interpreted satellite image (Google earth) with statistical orientation of

1023

faults (mineralized or not) and bedding. Stereonets (equal angle, lower hemisphere) show

1024

poles to faults and downdip lines of bedding; rose diagrams show strike orientation. See Fig. 3

45

ACCEPTED MANUSCRIPT 1025

for location and Fig. 2 for stratigraphic symbols. – B: View of the southeastern flank of the

1026

massif (see A for location).

1027

Fig. 7. The Bou Mayz anticline and associated egg-box pattern (interpreted satellite images

1029

Google earth). See Fig. 3 for location and Fig. 2 for stratigraphic symbols. - A: Western

1030

pericline of the main fold and interfering transverse folds. Anticlinal axis symbols as Fig. 5.

1031

SOF: South Ottara fault. – B: Eastern pericline; notice its dextral twist and associated

1032

fracturation (most faults mineralized in barite).

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Fig. 8. The Shayb Arras anticline and bordering synclines (see Fig. 3 for location and Fig. 2

1035

for stratigraphic symbols). – A: Interpreted satellite image (Google Earth) with location of

1036

figs. B-F. Thick dashed lines: first and second order anticline axes; thin dashed lines:

1037

synclines. Insert: statistical orientation of downdip lines of bedding (equal angle, lower

1038

hemisphere). – B: View of second order chevron folds in the northern limb Middle Devonian

1039

formations. – C: Minor folds in the Eifelian limestones detached over the Upper Emsian marls

1040

of the southern limb. – D: Partial zoom of the southeastern pericline showing the occurrence

1041

of several thrust and strike-slip faults. – E: View of the strike-slip sinistral corridor south of

1042

the Amessoui syncline. – F: Northern vertical wall of the ENE-trending « Filon 3 » barite vein

1043

of Shayb Arras, entirely mined. Notice the conspicuous horizontal slickensides with sinistral

1044

kinematic indicators.

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46

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Fig. 9. Western pericline of the Ottara syncline (interpreted satellite image; see Fig. 3 for

1047

location and Fig. 2 for stratigraphic symbols). Notice the transverse faults that correspond to

1048

totally or partially inverted Devonian paleofaults.

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Fig. 10. The “Gordian knot” of the Tadaout Massif. The interest of this complicated area is to

1051

expose N-trending transverse folds superimposed on major latitudinal folds. Landsat image N

1052

30-30-2000 interpreted with the help of the satellite images Google earth and the geological

1053

map 1:50,000, sheet Al Atrous (Benharref et al., in press). See Fig. 3 for location and Fig. 2

1054

for stratigraphic symbols. Anticlinal axis symbols: full circle for main early folds; empty

1055

circle for transverse, late folds. AMF: Amessoui-Mech Agrou fault; BHF: Bou Hmid fault;

1056

OJTF: Oumjerane-Taouz fault; OZF: Oued Ziz fault; TCF: Tadaout central fault; TRF; Tizi

1057

n’Ressas fault. Framed: Fig. 11.

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Fig. 11. The OJTF south of the Tadaout Massif (interpreted Google earth image). Location:

1060

Fig. 10. Stratigraphic symbols as Fig. 3. Notice the Lower Ordovician vertical beds in the

1061

tectonic lenses pinched between the massif and the Devonian-Carboniferous corridor that

1062

marks the OJTF fault zone. The coexistence of opposite senses of strike-slip suggests an

1063

important shortening component normal to the fault zone.

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Fig. 12. Structure of the Maider Basin (interpreted Google earth satellite image). Location:

1066

see Fig. 3. Stratigraphic symbols as Fig. 2. Anticlinal axis symbols as Fig. 10. EMF: East

1067

Maider Fault; ESF: East Signit Fault; NMF: North Mecissi Fault; OJTF: Oumjerane-Taouz

47

ACCEPTED MANUSCRIPT 1068

fault; SMF: South Mecissi Fault. The western side of the downwarped basin is a flexure zone

1069

with slowly eastward steepening dips.

1070

Fig. 13. Main structures of the South Tafilalt Basin. A: interpreted Google earth image.

1072

Location: see Fig. 3. Stratigraphic symbols as Fig. 2, structural symbols as Fig. 10. D (south

1073

of Widane Chebbi) : dolerite of probable Triassic-Liassic age. OZF: Oued Ziz Fault. Insert:

1074

statistical orientation of downdip lines of bedding (equal angle, lower hemisphere). – B: Mfis

1075

mineralized fault (see location in A).

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Fig. 14. Structural map of the South Tafilalt-East Maider area, based on satellite imagery

1078

(Google Earth) interpreted with the geological map of Morocco, scale 1:200,000 (Destombes

1079

& Hollard, 1986), the explanatory notices of the South Tafilalt 1:50,000 maps (Benharref et

1080

al., in press) and personal observations. See Fig. 3 for location and fault names. Notice, i) the

1081

contrast between the Kem Kem domain, south of the OJTF, and the others in the north; ii) the

1082

dominant sigmoidal axes of most anticlines and synclines of the Ougnat-Ouzina Axis (e.g.

1083

Shayb Arras anticline, Amessoui syncline, etc.), and iii) the occurrence of clear interference

1084

patterns by place (e.g. western Bou Mayz anticline, Tadaout massif etc.). The remarkable

1085

shapes of the southernmost anticlinal massifs of the Ougnat-Ouzina Axis (i.e. the croissant-

1086

shaped Tijekht anticline and the “Gordian knot” of the J. Tadaout) are best explained by

1087

superimposed folding events (see text and Fig. 18).

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48

ACCEPTED MANUSCRIPT Fig. 15. Devonian paleofaults in the Eastern Anti-Atlas. A: Restored Devonian paleofault

1090

pattern, after Baidder et al. (2008), modified. – B: Unconformity of the late Upper Devonian

1091

Aoufilal Sandstones (d7c) on the Upper Ordovician Tiouririne (or5c) and Upper Ktaoua

1092

(or6a) Fms. at Jdaid, southern bank of Oued Ziz. These outcrops are located south of the

1093

OJTF and contrast with the J. Mraier outcrops 7 km in the north, which include a complete,

1094

about 700 m-thick Upper Ordovician-Upper Devonian sequence.

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Fig. 16. Semi-interpretative, unbalanced cross-section of the Maider-South Tafilalt structures.

1097

For location, see Fig. 14. Stratigraphic symbols as Fig. 2. Vertical scale exaggerated twice

1098

with respect to horizontal scale. The cross-section has been drawn according to the near

1099

surface structures (above the dotted horizontal line) and respecting the thickness of the

1100

competent formations at deeper depth. The structures of the Ougnat-Ouzina Axis (Tijekht and

1101

Shayb Arras anticlines) are the most realistic. Within the South Tafilalt Basin, the structures

1102

are featured at depth schematically projecting the Mfis anticline axially to the NE, but they

1103

preserve the near surface structure of the Carboniferous formations. The structure of the

1104

Erfoud Anticlinorium is schematic as the trace of the cross-section is strongly oblique to the

1105

folds in this area, which moreover extends outside the domain we mapped in detail. The main

1106

stratigraphic variations from SW to NE are shown, i.e., i) the disappearance of the Early

1107

Cambrian deposits to the NE; ii) the big thickness variations of the Middle Devonian-

1108

Carboniferous deposits from the basins to the bordering highs. The importance of the strike-

1109

slip movements from block to block precludes balancing strictly the geological profile.

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Fig. 17. Fabric of the South Tafilalt folded rocks. – A: Upper Visean rocks of the Itima Fm.

1112

From the easternmost Marzouga synclinorium (Courtesy A. Tahiri, Rabat). – B: Compaction 49

ACCEPTED MANUSCRIPT of the sandy-argillaceous deposits of the Tournaisian deposits (Znaigui Fm) north of the Mfis

1114

anticline. – C: Vertical spaced cleavage in folded, olistolites-bearing Znaigui Fm in the

1115

Oumjerane-Taouz fault zone south of Taouz (see panorama Fig. 4C). – D: Sandstone beds and

1116

interleaved clay at the transition between the Schistes à Paradoxides and Tabanit formations

1117

in the eroded crest of the Tijekht anticline western corner. B and C from Benharref et al.

1118

(2014).

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Fig. 18. Cartoon of the evolution of the South Tafilalt structures as the result of two

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successive folding events “D1” and “D2” (steps B and C, respectively) applied to a mosaic of

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tilted blocks (step A). Consistently, the distribution of poles to bedding (D) is strongly

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scattered, although it shows two faint maxima oriented WNW and ENE. Compare with the

1125

central and lower part of the structural map (Fig. 14). AMF: Amessoui-Mech Agrou Fault;

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EMF: East Maider Fault; ESF: East Signit Fault; OJTF: Oumjerane-Taouz Fault; OZF: Oued

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Ziz Fault; STF: South Tijekht Fault; TRF: Tizi n’Ressas Fault.

TE D

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Highlights Anti-Atlas and Ougarta Variscan belts connect obliquely in the Tafilalt-Maider area

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Fold interferences there occur in a thick-skinned tectonic regime

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Fold trend is controlled both by basement faults and superposed stress events

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Paleofault inversion and stress rotation account for a croissant-shaped anticline

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Opening of barite veins postdates the Variscan folding

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